This invention relates to semiconductor quantized layered structures, particularly as applied to luminescent devices, such as semiconductor injection lasers and light emitting diodes.
In recent years, interest has intensified to the employment of quantum-mechanical effects in electrical properties provided in superlattice and other quantized layered structures formed by a periodic variation of alloy composition and/or doping impurity with a period less than the electron mean free path. The interest in this subject has been much enhanced, in part, by the development of molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MO-CVD) and automated liquid phase epitaxy (LPE). In these processes, very thin uniform layers of alloy composition comprising elements in Group III and Group V can be readily deposited so as to form one or more layers that alternate in bandgap properties and form at their interfaces a semiconductor heterojunction. The layer or alternating layers of a first material has or have an alloy composition that provides a direct, narrow bandgap and, therefore, efficient light emitting capability, compared to the alternating layers of a second material having an alloy composition that provides a wider bandgap than that of the first material. Many times, this wider bandgap material has an indirect band structure. As a result, the conduction bands of the second material have different energy levels so that the indirect band is at a lower energy level compared to the direct band whereas the conduction bands of the first material have different energy levels so that the direct band is at a lower level compared to the indirect band. The direct bandgap property of the first material permits the relatively easy attainment of carrier recombination and emission of photons which, combined with further stimulation, can produce lasing conditions.
Conversely, the indirect bandgap structure of the second material does not provide for efficient radiative recombination since the recombination of an electron and hole involves the simultaneous emission of both a photon and a phonon which is very improbable. In a quantized layered structure, the layer or layers of the first material being thinner then the electron mean free path, form a one-dimensional periodic potential medium comprising a plurality of potential wells in the first material and barriers formed by the second layers. These wells and barriers quantize and trap electrons, leading to an increase in the energy emitted by each photon in the direct bandgap first material. Background in this subject may be found in U.S. Pat. No. 4,261,771 and the references cited in this patent.
Recently studies have been conducted to determine the nature of phonon contribution in heterostructure laser operation relative to both direct and indirect gap levels. One example is the article of B. A. Vojak, N. Holonyak, Jr., W. D. Laidig, K. Hess, J. J. Coleman and P. D. Dapkus entitled "Phonon Contribution to Metalorganic Chemical Vapor Deposited Al.sub.x Ga.sub.1-x AsGaAs Quantum-Well Heterostructure Laser Operation", Journal of Applied Physics, Vol. 52(2), pp 959-968, February, 1981. By understanding phonon contributions, it may be possible to improve the luminescence efficiency of light emitting materials having major indirect bands, i.e., a conduction band minimum that lie at a different position in momentum space (k-space) than does the valence band maximum.
The suggestion has been made that the luminescence efficiency could be improved in alloy compositions in the Group III-V elements having a lower energy level indirect bandgap compared to the direct bandgap by shifting the indirect bandgap to be a direct bandgap material. See U.S. Pat. No. 3,872,400. This direct bandgap transition is said to be created by alternating layers of different alloy or nonalloy compositions.
As a more viable alternative, direct bandgap type of transition for indirect bandgap semiconductor materials may be achieved by quantum wells and barriers in layers of indirect bandgap materials to quantize electron states rather than shift the indirect bandgap to the point of zero momentum space to improve luminescence efficiency. Also, the incorporation of isoelectronic centers in indirect bandgap semiconductor materials forming quantum well layers provides for a three dimensional potential structure for quantizing electron states.